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Fabrication of β-TCP foam: Effects of magnesium oxide as phase stabilizer on its properties
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 14245–14250 www.elsevier.com/locate/ceramint
Fabrication of β-TCP foam: Effects of magnesium oxide as phase stabilizer on its properties Taro Nikaidoa,b, Kanji Tsurua,n, Melvin Munara, Michito Marutac, Shigeki Matsuyac, Seiji Nakamurab, Kunio Ishikawaa a Department of Biomaterials, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan Section of Oral and Maxillofacial Oncology, Division of Maxillofacial Diagnostic and Surgical Sciences, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan c Section of Bioengineering, Department of Dental Engineering, Fukuoka Dental Collage, 2-15-1 Tamura, Sawara-ku, Fukuoka 814-0193, Japan
b
Received 7 May 2015; received in revised form 6 July 2015; accepted 9 July 2015 Available online 16 July 2015
Abstract Porous β-tricalcium phosphate (β-TCP) has attracted attention as an artificial bone substitute. α-TCP foam that mimics the fully interconnected porous structure of the cancellous bone can be fabricated by a ceramics foam method, which requires sintering at high temperature. However, βTCP foam could not be fabricated by the ceramics foam method since the sintering temperature required is above the αβ transition temperature. To fabricate the β-TCP foam that mimics the fully interconnected porous structure of cancellous bone, magnesium oxide (MgO) is added as a phase stabilizer. β-TCP foam was obtained when at least 3 mol% MgO was added. Lattice parameter analysis revealed that the MgO as a β-phase stabilizer was incorporated into the lattice of the TCP structure. The porosity of the β-TCP foam containing 3 mol% MgO was 92%, and the compressive strength was approximately 32 kPa. Despite the observed smooth surface and lower porosity of β-TCP foam compared to α-TCP foam, no improvement on the compressive strength of β-TCP foam occurred even by the addition of 3 mol% or more of MgO. Formation of micro-cracks could be one the reasons. These micro-cracks were formed by contraction as a result from growth of adjacent grains. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Bone replacement; Porous materials; β-ΤΧΠ; Magnesium oxide
1. Introduction Hydroxyapatite (HAp: Ca10(PO4)6(OH)2) and β-tricalcium phosphate (β-TCP: Ca3(PO4)2) have been used clinically as artificial bone substitutes because of their excellent tissue compatibility and osteoconductivity [1–4]. While HAp is stable, β-TCP is biodegradable and promotes new bone growth. It has been reported that the pore size and structure of β-TCP play an important role in promoting bone formation [5–8]. Among the many methods for the fabrication of porous ceramics, the so-called ceramic foam method or polyurethane foam replica method is unique in that it produces an interconnected porous structure similar to cancellous bone n
Corresponding author. Tel.: þ81 92 642 6345; fax: þ 81 92 642 6348. E-mail address: [email protected] (K. Tsuru).
http://dx.doi.org/10.1016/j.ceramint.2015.07.053 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
[9–13]. In this method, polyurethane foam used as a template is immersed in a ceramic slurry such that the ceramics powder coats the struts of the polyurethane foam. When the polyurethane foam coated with ceramics powder is heated in a furnace, both the sintering of the ceramics powder and burning out of the polyurethane foam occur simultaneously. Consequently, a ceramic foam similar to polyurethane foam or cancellous bone is obtained. Although the ceramic foam method allows the production of the desired interconnected porous structure, a compacting process is difficult to apply to the calcium phosphate-coated polyurethane foam. Thus, a relatively high temperature is required for the sintering process. Fabrication of HAp foam and α-tricalcium phosphate (α-TCP: Ca3(PO4)2) foam is much simpler as they are both stable at the high temperatures required for sintering. However, β-TCP foam cannot be fabricated using this method since
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sintering below the α,β-transition temperature (1180 1C) results in a β-TCP foam with very low mechanical strength. In fact, the foam could not be removed from the sintering furnace because of its weak structure owing to our preliminary trial in this study. Fortunately, magnesium oxide (MgO) has been reported to increase the thermal stability of β-TCP phase when it is used as a sintering additive [14–20]. The addition of MgO affects the structural stabilization as a result of the substitution of Mg ions into Ca sites and vacancy in β-TCP [21–22]. Furthermore, it should be pointed out that the presence of Mg2 þ ions is known to improve the biocompatibility and bioactivity of TCP ceramics in calcified tissue [23–26]. For these reasons, MgO is included into the starting materials and sintered at high temperature so as to achieve β-TCP foam. Therefore, the feasibility to fabricate β-TCP foam using MgO as phase stabilizer is investigated in this study.
X-ray diffractometer (D8 Advance, Bruker AXS GmbH., Karlsruhe, Germany) using monochromatized X-ray (CuKα: λ=0.1542 nm) operating at 40 kV tube voltage and 40 mA tube current. The diffraction angle was continuously scanned from 101 to 601 in 2θ at a scanning rate of 21/min. Only 25–351 is shown in Fig. 5 as no relevant peaks were observed in the excluded regions. 2.3. Linear shrinkage Linear shrinkage was determined by comparing the original size of the polyurethane foam template with the size of the obtained TCP foams after sintering. Length of a side for both polyurethane foam and the obtained TCP foam was measured by vernier calipers (VC-15, Niigata Seiki, Niigata, Japan) and the lengths used to calculate the linear shrinkage in percentage. Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD).
2. Materials and methods 2.1. Fabrication of β-TCP foam The polyurethane foam replica method was used to fabricate the β-TCP foam. Polyurethane foam (HR-20D, Bridgestone Corp., Tokyo, Japan) with an average pore size of approximately 1 mm in diameter was shaped to 20 20 20 mm3 and used as a template. Calcium phosphate slurry containing magnesium oxide (MgO, Wako Pure Chemical, Osaka, Japan) was made by mixing commercially obtained magnesium oxide, calcium carbonate, and dicalcium phosphate anhydrous (Wako Pure Chemical) powders. Seven powders were mixed so that the amount of MgO was 0, 1, 2, 3, 4, 6, and 8 mol% with a (Ca þ Mg)/P molar ratio of 1.5. The powder mixtures were made into slurry with distilled water containing 5 wt% polyvinyl alcohol (Ishizu Pharmaceutical, Osaka, Japan) at a powder/liquid ratio of 8 g/10 mL. The polyurethane foams were immersed in the prepared slurry and excess slurry was removed by squeezing. Then the slurry-coated polyurethane foams were dried in a drying oven (DO-300A, AS ONE Corp., Osaka, Japan) at 60 1C for at least 12 h, followed by sintering in a programmable electric furnace (SC-2035D, Motoyama, Osaka, Japan). The coated foams were heated up to 400 1C at a rate of 1 1C/min from room temperature to burn out the polyurethane foam, followed by heating at a rate of 5 1C/min from 400 1C to 1,500 1C. The foams were kept at 1500 1C for 5 h and allowed to cool down inside the furnace to room temperature. In the following, the molar percentage of Mg in the TCP foam is stated in parentheses. For example, TCP(3) represents TCP containing 3 mol% Mg. 2.2. Powder X-ray diffraction The crystal phase of the TCP foams was analyzed using powder X-ray diffraction (XRD) analysis. The obtained TCP foams were ground into fine powder and supplied for the analysis. XRD patterns of the specimens were recorded with an
2.4. Scanning electron microscopy The surface morphology of the TCP foams was observed by a scanning electron microscope (SEM, S-3400N, Hitachi HighTechnologies Co., Tokyo, Japan) at 15 kV of accelerating voltage after gold–palladium coating by a magnetron sputtering machine (MSP-1S, Vacuum Device Co., Ibaraki, Japan). 2.5. Porosity The porosity of the TCP foams was calculated using the following equation: Porosity of TCP foam ¼ 100 ðd TCP dTCP foam Þ=dTCP where dTCP is the true density of the TCP foam, measured by a pycnometer flask; and dTCP foam is the bulk density of the TCP foam, calculated as the weight-to-volume ratio. Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD). 2.6. Mechanical strength measurement The mechanical strength of the TCP foam was evaluated in terms of compressive strength. Compressive strength was measured by crashing the foam at a crosshead speed of 1 mm/min using a universal testing machine (Autograph AGS-J, Shimadzu Corporation, Kyoto, Japan). Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD). 3. Results Fig. 1 shows the typical macroscopic structure of (a) the polyurethane foam template and (b–h) the TCP foams prepared using calcium phosphate slurry containing (b) 0 mol%, (c) 1 mol%, (d) 2 mol%, (e) 3 mol%, (f) 4 mol%, (g) 6 mol%, and (h) 8 mol% MgO. As shown in this figure, the TCP foams showed the same interconnected porous structure of
T. Nikaido et al. / Ceramics International 41 (2015) 14245–14250
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Fig. 1. Typical macroscopic structure of (a) the polyurethane foam template and (b–h) TCP foams prepared using calcium phosphate slurry containing (b) 0 mol%, (c) 1 mol%, (d) 2 mol%, (e) 3 mol%, (f) 4 mol%, (g) 6 mol% and (h) 8 mol% MgO.
Fig. 2. Linear shrinkage of the TCP foams containing different amounts of MgO. (n ¼10, n: po0.01, compared to TCP(0), see text for sample code.).
polyurethane foam regardless of the absence or presence of MgO or the molar content of MgO. The size of the TCP foams was smaller when compared to polyurethane foam used as a template. Fig. 2 summarizes the linear shrinkage of the TCP foams containing different molar percentages of MgO. Linear shrinkage was 25% for TCP(0) and TCP(1). Linear shrinkage significantly increased with added MgO to reach a maximum value of 35% when 3 mol% MgO was added to TCP (n¼ 10, n: po 0.01, compared to TCP(0)). However, linear shrinkage did not increase as the molar percentage of MgO was further increased up to 8 mol%. Fig. 3 shows typical photographs of (a) the polyurethane foam used as a template, (b) TCP(0), and (c) TCP(3). The photograph with higher magnification further confirms that the same interconnected porous structure is present in both the (a) polyurethane foam and (b, c) TCP foams. The pore size of the TCP foams was smaller than that of the polyurethane foam, and the pore size of TCP(3), approximately 650 mm, was smaller than that of TCP(0), approximately 750 mm. Fig. 4 shows the typical SEM images of (a, h) TCP(0), (b, i) TCP(1), (c, j) TCP(2), (d, k) TCP(3), (e, l) TCP(4), (f, m) TCP (6), and (g, n) TCP(8). When the MgO content of α-TCP foam was 3 mol% or larger, the surface of the TCP foam struts was smooth. SEM observation under high magnification indicated that the grain size increased with added MgO and reached 10– 30 μm when 4 mol% MgO was added. Then the size became constant up to 8 mol% MgO. Furthermore, micro-cracks were observed in TCP(3), TCP(4), TCP(6), and TCP(8) as indicated by arrows in Fig. 4(k)–(n), whereas TCP(0), TCP(1), and TCP
(2) were nearly micro-crack free, although intergranular spaces existed. Fig. 5 summarizes the powder XRD patterns of (b) TCP(0), (c) TCP(1), (d) TCP(2), (e) TCP(3), (f) TCP(4), (g) TCP(6), and (h) TCP(8). XRD patterns of commercially obtained (a) αTCP and (i) β-TCP powders are also shown for comparison. The crystal phase of TCP(0) and TCP(1) was identical to only the α-TCP phase. In contrast, the crystal phase of TCP(3), TCP (4), TCP(6), and TCP(8) was single phase of β-TCP. In the case of TCP(2), the crystal phase was a mixture of α- and βTCP. No peaks corresponding to MgO were detected in the XRD patterns even for the foam prepared by addition of 8 mol % MgO. Fig. 6 summarizes the lattice parameters of a- and c-axes for pure β-TCP, TCP(2), TCP(3), TCP(4), TCP(6), and TCP(8). Both the a- and c-axes became shorter as the amount of added MgO increased. Fig. 7 shows the compressive strength of the TCP foams containing different amounts of MgO. The compressive strength was approximately 32 kPa regardless of the absence and presence of the MgO or the amount of added MgO. Fig. 8 summarizes porosity of the TCP foams containing different amounts of MgO. The porosity was above 90% for all specimens, regardless of the amount of added MgO. The αTCP foams containing 3 mol% or more MgO showed smaller porosity as compared to TCP(0) (n ¼ 10, po 0.01). 4. Discussion Results obtained in this study clearly demonstrate that 3 mol% MgO-containing calcium phosphate powder with (Ca þ Mg)/P ¼ 1.5 is useful for the fabrication of β-TCP foam. When polyurethane foam coated with the powder was sintered at 1500 1C, β-TCP foam was fabricated. Since the α,βtransition temperature of TCP is 1180 1C, it is apparent that the added MgO stabilized the β-TCP phase. The lattice parameters of the a- and c-axes decreased with an increase in the amount of added MgO as shown in Fig. 6. These results indicate that Mg is incorporated into the lattice structure of TCP; these results are in agreement with previous reports [17–19]. When 1 mol% MgO was added to TCP, its effect was not enough to stabilize the β-phase at 1500 1C. Addition of 2 mol% MgO was still not enough, and thus both α- and β-phases appeared in the TCP foam. However, when 3 mol% or more MgO was added to the TCP foam, only the β-TCP
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Fig. 3. Typical photographs of (a) the polyurethane foam used as template, (b) TCP(0), and (c) TCP(3).
Fig. 4. Typical SEM images of (a, h) TCP(0), (b, i) TCP(1), (c, j) TCP(2), (d, k) TCP(3), (e, l) TCP(4), (f, m) TCP(6), and (g, n) TCP(8). Upper side: low magnification, lower side: high magnification. Arrows indicate micro-cracks.
phase was obtained. In other words, 3 mol% or more MgO was required to stabilize β-TCP at 1500 1C. These results are similar to the results reported by Enderle et al. who reported that the stabilization of the β-TCP structure required at least 4 mol% or more MgO when sintered at 1500 1C [17]. Linear shrinkage, pore size, and porosity of the foam were dependent on the phase of TCP as shown in Figs. 2, 3, and 8. Note that β-TCP (3.07 g/cm3) has a higher density than α-TCP (2.86 g/cm3) [27]. Therefore, the linear shrinkages of the βTCP foams TCP(3), TCP(4), TCP(6), and TCP(8) – are larger than the α-TCP foams TCP(0) and TCP(1). Furthermore, the linear shrinkage of the α-, β-TCP foam TCP(2) was between that of the α- and β-TCP foams. β-TCP foams TCP(3), TCP(4), TCP(6) and TCP(8) show larger linear shrinkage resulting to
smaller pore size and porosity compared to α-TCP foams TCP (0) and TCP(1) as shown in Figs. 2, 3 and 8. As shown in Fig. 4, the surface of TCP became smoother with the amount of added MgO. This is thought to be the result of MgO acting as a sintering additive [14,28]. Mechanical strength usually increases when the porosity decreases (as shown in Fig. 8) and the surface becomes smooth as shown in Fig. 4. Interestingly, however, mechanical strength between αand β-TCP foams was almost the same (as shown in Fig. 7), despite the observed smooth surface and lower porosity of βTCP foam compared with the α-TCP foam (Figs. 4 and 8). One of the reasons might be the formation of micro-cracks that were confirmed to be present by SEM observation under high magnification as shown in Fig. 4(k)–(n). It has been reported
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Fig. 7. Compressive strength of the TCP foams containing different amounts of MgO. XRD analysis indicated that β-TCP foams could be obtained by the addition of more than 3 mol% MgO. (n ¼10, no statistical difference among data.).
Fig. 5. Powder XRD patterns of (b) TCP(0), (c) TCP(1), (d) TCP(2), (e) TCP (3), (f) TCP(4), (g) TCP(6), and (h) TCP(8). XRD patterns of commercially obtained (a) α-TCP powder and (i) β-TCP powder are also shown for comparison. ▼: α-TCP, ▽: β-TCP.
Fig. 8. Porosity of the TCP foams containing different amounts of MgO. XRD analysis indicated that β-TCP foams could be obtained by the addition of more than 3 mol% MgO. (n¼10, n: po0.01, compared to TCP(0)).
Fig. 6. Lattice parameters of the a- and c-axes for pure β-TCP, TCP(2), TCP (3), TCP(4), TCP(6) and TCP(8). Lattice parameter a:◯, b:●.
that these micro-cracks are formed by contraction due to growth of grains adjacent to each other [28]. As a result of the formation of these micro-cracks, the mechanical strength of TCP could not be improved by the addition of MgO despite the smoother surface and lower porosity. β-TCP foam with fully interconnected pore structure and high porosity approximately 92%—was first fabrication in the course of the present study. Therefore, a future in vivo study in which a bone defect is reconstructed using β-TCP foam fabricated in the present study is anticipated.
required to stabilize the β-TCP phase at 1500 1C. The compressive strength of β-TCP foam was approximately 32 kPa and the porosity was as high as 92%. Although it was expected that the effect of MgO addition as sintering additive would improve the compressive strength of β-TCP foam, it was counteracted by the appearance of micro-cracks formed by contraction due to grain growth.
Acknowledgments This study was supported in part by the Strategic Promotion of Innovative Research (Grant No. AJ120495) and Development Program of the Japan Agency for Medical Research (Grant No. 25893175).
References 5. Conclusion The present study demonstrated that β-TCP foam can be prepared by the polyurethane foam replica method using MgO as β-TCP phase stabilizer. Three mol% or more MgO was
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 14245–14250 www.elsevier.com/locate/ceramint
Fabrication of β-TCP foam: Effects of magnesium oxide as phase stabilizer on its properties Taro Nikaidoa,b, Kanji Tsurua,n, Melvin Munara, Michito Marutac, Shigeki Matsuyac, Seiji Nakamurab, Kunio Ishikawaa a Department of Biomaterials, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan Section of Oral and Maxillofacial Oncology, Division of Maxillofacial Diagnostic and Surgical Sciences, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan c Section of Bioengineering, Department of Dental Engineering, Fukuoka Dental Collage, 2-15-1 Tamura, Sawara-ku, Fukuoka 814-0193, Japan
b
Received 7 May 2015; received in revised form 6 July 2015; accepted 9 July 2015 Available online 16 July 2015
Abstract Porous β-tricalcium phosphate (β-TCP) has attracted attention as an artificial bone substitute. α-TCP foam that mimics the fully interconnected porous structure of the cancellous bone can be fabricated by a ceramics foam method, which requires sintering at high temperature. However, βTCP foam could not be fabricated by the ceramics foam method since the sintering temperature required is above the αβ transition temperature. To fabricate the β-TCP foam that mimics the fully interconnected porous structure of cancellous bone, magnesium oxide (MgO) is added as a phase stabilizer. β-TCP foam was obtained when at least 3 mol% MgO was added. Lattice parameter analysis revealed that the MgO as a β-phase stabilizer was incorporated into the lattice of the TCP structure. The porosity of the β-TCP foam containing 3 mol% MgO was 92%, and the compressive strength was approximately 32 kPa. Despite the observed smooth surface and lower porosity of β-TCP foam compared to α-TCP foam, no improvement on the compressive strength of β-TCP foam occurred even by the addition of 3 mol% or more of MgO. Formation of micro-cracks could be one the reasons. These micro-cracks were formed by contraction as a result from growth of adjacent grains. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Bone replacement; Porous materials; β-ΤΧΠ; Magnesium oxide
1. Introduction Hydroxyapatite (HAp: Ca10(PO4)6(OH)2) and β-tricalcium phosphate (β-TCP: Ca3(PO4)2) have been used clinically as artificial bone substitutes because of their excellent tissue compatibility and osteoconductivity [1–4]. While HAp is stable, β-TCP is biodegradable and promotes new bone growth. It has been reported that the pore size and structure of β-TCP play an important role in promoting bone formation [5–8]. Among the many methods for the fabrication of porous ceramics, the so-called ceramic foam method or polyurethane foam replica method is unique in that it produces an interconnected porous structure similar to cancellous bone n
Corresponding author. Tel.: þ81 92 642 6345; fax: þ 81 92 642 6348. E-mail address: [email protected] (K. Tsuru).
http://dx.doi.org/10.1016/j.ceramint.2015.07.053 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
[9–13]. In this method, polyurethane foam used as a template is immersed in a ceramic slurry such that the ceramics powder coats the struts of the polyurethane foam. When the polyurethane foam coated with ceramics powder is heated in a furnace, both the sintering of the ceramics powder and burning out of the polyurethane foam occur simultaneously. Consequently, a ceramic foam similar to polyurethane foam or cancellous bone is obtained. Although the ceramic foam method allows the production of the desired interconnected porous structure, a compacting process is difficult to apply to the calcium phosphate-coated polyurethane foam. Thus, a relatively high temperature is required for the sintering process. Fabrication of HAp foam and α-tricalcium phosphate (α-TCP: Ca3(PO4)2) foam is much simpler as they are both stable at the high temperatures required for sintering. However, β-TCP foam cannot be fabricated using this method since
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sintering below the α,β-transition temperature (1180 1C) results in a β-TCP foam with very low mechanical strength. In fact, the foam could not be removed from the sintering furnace because of its weak structure owing to our preliminary trial in this study. Fortunately, magnesium oxide (MgO) has been reported to increase the thermal stability of β-TCP phase when it is used as a sintering additive [14–20]. The addition of MgO affects the structural stabilization as a result of the substitution of Mg ions into Ca sites and vacancy in β-TCP [21–22]. Furthermore, it should be pointed out that the presence of Mg2 þ ions is known to improve the biocompatibility and bioactivity of TCP ceramics in calcified tissue [23–26]. For these reasons, MgO is included into the starting materials and sintered at high temperature so as to achieve β-TCP foam. Therefore, the feasibility to fabricate β-TCP foam using MgO as phase stabilizer is investigated in this study.
X-ray diffractometer (D8 Advance, Bruker AXS GmbH., Karlsruhe, Germany) using monochromatized X-ray (CuKα: λ=0.1542 nm) operating at 40 kV tube voltage and 40 mA tube current. The diffraction angle was continuously scanned from 101 to 601 in 2θ at a scanning rate of 21/min. Only 25–351 is shown in Fig. 5 as no relevant peaks were observed in the excluded regions. 2.3. Linear shrinkage Linear shrinkage was determined by comparing the original size of the polyurethane foam template with the size of the obtained TCP foams after sintering. Length of a side for both polyurethane foam and the obtained TCP foam was measured by vernier calipers (VC-15, Niigata Seiki, Niigata, Japan) and the lengths used to calculate the linear shrinkage in percentage. Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD).
2. Materials and methods 2.1. Fabrication of β-TCP foam The polyurethane foam replica method was used to fabricate the β-TCP foam. Polyurethane foam (HR-20D, Bridgestone Corp., Tokyo, Japan) with an average pore size of approximately 1 mm in diameter was shaped to 20 20 20 mm3 and used as a template. Calcium phosphate slurry containing magnesium oxide (MgO, Wako Pure Chemical, Osaka, Japan) was made by mixing commercially obtained magnesium oxide, calcium carbonate, and dicalcium phosphate anhydrous (Wako Pure Chemical) powders. Seven powders were mixed so that the amount of MgO was 0, 1, 2, 3, 4, 6, and 8 mol% with a (Ca þ Mg)/P molar ratio of 1.5. The powder mixtures were made into slurry with distilled water containing 5 wt% polyvinyl alcohol (Ishizu Pharmaceutical, Osaka, Japan) at a powder/liquid ratio of 8 g/10 mL. The polyurethane foams were immersed in the prepared slurry and excess slurry was removed by squeezing. Then the slurry-coated polyurethane foams were dried in a drying oven (DO-300A, AS ONE Corp., Osaka, Japan) at 60 1C for at least 12 h, followed by sintering in a programmable electric furnace (SC-2035D, Motoyama, Osaka, Japan). The coated foams were heated up to 400 1C at a rate of 1 1C/min from room temperature to burn out the polyurethane foam, followed by heating at a rate of 5 1C/min from 400 1C to 1,500 1C. The foams were kept at 1500 1C for 5 h and allowed to cool down inside the furnace to room temperature. In the following, the molar percentage of Mg in the TCP foam is stated in parentheses. For example, TCP(3) represents TCP containing 3 mol% Mg. 2.2. Powder X-ray diffraction The crystal phase of the TCP foams was analyzed using powder X-ray diffraction (XRD) analysis. The obtained TCP foams were ground into fine powder and supplied for the analysis. XRD patterns of the specimens were recorded with an
2.4. Scanning electron microscopy The surface morphology of the TCP foams was observed by a scanning electron microscope (SEM, S-3400N, Hitachi HighTechnologies Co., Tokyo, Japan) at 15 kV of accelerating voltage after gold–palladium coating by a magnetron sputtering machine (MSP-1S, Vacuum Device Co., Ibaraki, Japan). 2.5. Porosity The porosity of the TCP foams was calculated using the following equation: Porosity of TCP foam ¼ 100 ðd TCP dTCP foam Þ=dTCP where dTCP is the true density of the TCP foam, measured by a pycnometer flask; and dTCP foam is the bulk density of the TCP foam, calculated as the weight-to-volume ratio. Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD). 2.6. Mechanical strength measurement The mechanical strength of the TCP foam was evaluated in terms of compressive strength. Compressive strength was measured by crashing the foam at a crosshead speed of 1 mm/min using a universal testing machine (Autograph AGS-J, Shimadzu Corporation, Kyoto, Japan). Ten specimens were used for each condition and the results were expressed as mean (%) 7 standard deviation (SD). 3. Results Fig. 1 shows the typical macroscopic structure of (a) the polyurethane foam template and (b–h) the TCP foams prepared using calcium phosphate slurry containing (b) 0 mol%, (c) 1 mol%, (d) 2 mol%, (e) 3 mol%, (f) 4 mol%, (g) 6 mol%, and (h) 8 mol% MgO. As shown in this figure, the TCP foams showed the same interconnected porous structure of
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Fig. 1. Typical macroscopic structure of (a) the polyurethane foam template and (b–h) TCP foams prepared using calcium phosphate slurry containing (b) 0 mol%, (c) 1 mol%, (d) 2 mol%, (e) 3 mol%, (f) 4 mol%, (g) 6 mol% and (h) 8 mol% MgO.
Fig. 2. Linear shrinkage of the TCP foams containing different amounts of MgO. (n ¼10, n: po0.01, compared to TCP(0), see text for sample code.).
polyurethane foam regardless of the absence or presence of MgO or the molar content of MgO. The size of the TCP foams was smaller when compared to polyurethane foam used as a template. Fig. 2 summarizes the linear shrinkage of the TCP foams containing different molar percentages of MgO. Linear shrinkage was 25% for TCP(0) and TCP(1). Linear shrinkage significantly increased with added MgO to reach a maximum value of 35% when 3 mol% MgO was added to TCP (n¼ 10, n: po 0.01, compared to TCP(0)). However, linear shrinkage did not increase as the molar percentage of MgO was further increased up to 8 mol%. Fig. 3 shows typical photographs of (a) the polyurethane foam used as a template, (b) TCP(0), and (c) TCP(3). The photograph with higher magnification further confirms that the same interconnected porous structure is present in both the (a) polyurethane foam and (b, c) TCP foams. The pore size of the TCP foams was smaller than that of the polyurethane foam, and the pore size of TCP(3), approximately 650 mm, was smaller than that of TCP(0), approximately 750 mm. Fig. 4 shows the typical SEM images of (a, h) TCP(0), (b, i) TCP(1), (c, j) TCP(2), (d, k) TCP(3), (e, l) TCP(4), (f, m) TCP (6), and (g, n) TCP(8). When the MgO content of α-TCP foam was 3 mol% or larger, the surface of the TCP foam struts was smooth. SEM observation under high magnification indicated that the grain size increased with added MgO and reached 10– 30 μm when 4 mol% MgO was added. Then the size became constant up to 8 mol% MgO. Furthermore, micro-cracks were observed in TCP(3), TCP(4), TCP(6), and TCP(8) as indicated by arrows in Fig. 4(k)–(n), whereas TCP(0), TCP(1), and TCP
(2) were nearly micro-crack free, although intergranular spaces existed. Fig. 5 summarizes the powder XRD patterns of (b) TCP(0), (c) TCP(1), (d) TCP(2), (e) TCP(3), (f) TCP(4), (g) TCP(6), and (h) TCP(8). XRD patterns of commercially obtained (a) αTCP and (i) β-TCP powders are also shown for comparison. The crystal phase of TCP(0) and TCP(1) was identical to only the α-TCP phase. In contrast, the crystal phase of TCP(3), TCP (4), TCP(6), and TCP(8) was single phase of β-TCP. In the case of TCP(2), the crystal phase was a mixture of α- and βTCP. No peaks corresponding to MgO were detected in the XRD patterns even for the foam prepared by addition of 8 mol % MgO. Fig. 6 summarizes the lattice parameters of a- and c-axes for pure β-TCP, TCP(2), TCP(3), TCP(4), TCP(6), and TCP(8). Both the a- and c-axes became shorter as the amount of added MgO increased. Fig. 7 shows the compressive strength of the TCP foams containing different amounts of MgO. The compressive strength was approximately 32 kPa regardless of the absence and presence of the MgO or the amount of added MgO. Fig. 8 summarizes porosity of the TCP foams containing different amounts of MgO. The porosity was above 90% for all specimens, regardless of the amount of added MgO. The αTCP foams containing 3 mol% or more MgO showed smaller porosity as compared to TCP(0) (n ¼ 10, po 0.01). 4. Discussion Results obtained in this study clearly demonstrate that 3 mol% MgO-containing calcium phosphate powder with (Ca þ Mg)/P ¼ 1.5 is useful for the fabrication of β-TCP foam. When polyurethane foam coated with the powder was sintered at 1500 1C, β-TCP foam was fabricated. Since the α,βtransition temperature of TCP is 1180 1C, it is apparent that the added MgO stabilized the β-TCP phase. The lattice parameters of the a- and c-axes decreased with an increase in the amount of added MgO as shown in Fig. 6. These results indicate that Mg is incorporated into the lattice structure of TCP; these results are in agreement with previous reports [17–19]. When 1 mol% MgO was added to TCP, its effect was not enough to stabilize the β-phase at 1500 1C. Addition of 2 mol% MgO was still not enough, and thus both α- and β-phases appeared in the TCP foam. However, when 3 mol% or more MgO was added to the TCP foam, only the β-TCP
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Fig. 3. Typical photographs of (a) the polyurethane foam used as template, (b) TCP(0), and (c) TCP(3).
Fig. 4. Typical SEM images of (a, h) TCP(0), (b, i) TCP(1), (c, j) TCP(2), (d, k) TCP(3), (e, l) TCP(4), (f, m) TCP(6), and (g, n) TCP(8). Upper side: low magnification, lower side: high magnification. Arrows indicate micro-cracks.
phase was obtained. In other words, 3 mol% or more MgO was required to stabilize β-TCP at 1500 1C. These results are similar to the results reported by Enderle et al. who reported that the stabilization of the β-TCP structure required at least 4 mol% or more MgO when sintered at 1500 1C [17]. Linear shrinkage, pore size, and porosity of the foam were dependent on the phase of TCP as shown in Figs. 2, 3, and 8. Note that β-TCP (3.07 g/cm3) has a higher density than α-TCP (2.86 g/cm3) [27]. Therefore, the linear shrinkages of the βTCP foams TCP(3), TCP(4), TCP(6), and TCP(8) – are larger than the α-TCP foams TCP(0) and TCP(1). Furthermore, the linear shrinkage of the α-, β-TCP foam TCP(2) was between that of the α- and β-TCP foams. β-TCP foams TCP(3), TCP(4), TCP(6) and TCP(8) show larger linear shrinkage resulting to
smaller pore size and porosity compared to α-TCP foams TCP (0) and TCP(1) as shown in Figs. 2, 3 and 8. As shown in Fig. 4, the surface of TCP became smoother with the amount of added MgO. This is thought to be the result of MgO acting as a sintering additive [14,28]. Mechanical strength usually increases when the porosity decreases (as shown in Fig. 8) and the surface becomes smooth as shown in Fig. 4. Interestingly, however, mechanical strength between αand β-TCP foams was almost the same (as shown in Fig. 7), despite the observed smooth surface and lower porosity of βTCP foam compared with the α-TCP foam (Figs. 4 and 8). One of the reasons might be the formation of micro-cracks that were confirmed to be present by SEM observation under high magnification as shown in Fig. 4(k)–(n). It has been reported
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Fig. 7. Compressive strength of the TCP foams containing different amounts of MgO. XRD analysis indicated that β-TCP foams could be obtained by the addition of more than 3 mol% MgO. (n ¼10, no statistical difference among data.).
Fig. 5. Powder XRD patterns of (b) TCP(0), (c) TCP(1), (d) TCP(2), (e) TCP (3), (f) TCP(4), (g) TCP(6), and (h) TCP(8). XRD patterns of commercially obtained (a) α-TCP powder and (i) β-TCP powder are also shown for comparison. ▼: α-TCP, ▽: β-TCP.
Fig. 8. Porosity of the TCP foams containing different amounts of MgO. XRD analysis indicated that β-TCP foams could be obtained by the addition of more than 3 mol% MgO. (n¼10, n: po0.01, compared to TCP(0)).
Fig. 6. Lattice parameters of the a- and c-axes for pure β-TCP, TCP(2), TCP (3), TCP(4), TCP(6) and TCP(8). Lattice parameter a:◯, b:●.
that these micro-cracks are formed by contraction due to growth of grains adjacent to each other [28]. As a result of the formation of these micro-cracks, the mechanical strength of TCP could not be improved by the addition of MgO despite the smoother surface and lower porosity. β-TCP foam with fully interconnected pore structure and high porosity approximately 92%—was first fabrication in the course of the present study. Therefore, a future in vivo study in which a bone defect is reconstructed using β-TCP foam fabricated in the present study is anticipated.
required to stabilize the β-TCP phase at 1500 1C. The compressive strength of β-TCP foam was approximately 32 kPa and the porosity was as high as 92%. Although it was expected that the effect of MgO addition as sintering additive would improve the compressive strength of β-TCP foam, it was counteracted by the appearance of micro-cracks formed by contraction due to grain growth.
Acknowledgments This study was supported in part by the Strategic Promotion of Innovative Research (Grant No. AJ120495) and Development Program of the Japan Agency for Medical Research (Grant No. 25893175).
References 5. Conclusion The present study demonstrated that β-TCP foam can be prepared by the polyurethane foam replica method using MgO as β-TCP phase stabilizer. Three mol% or more MgO was
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